Television signal translating circuit



Nov. 21,1944. Y R..B. DOME v I 2,363,299

TELEVISION SIGNAL TRANSLATING CIRCUIT Filed NOV. 1, 1943 PICTURE SIGNAL OUTPU 7' 4- 28 I SYNCHRON/Z/NG PULSES lnvehcori Robert Dome, by 1/ M v His ttorney.

Patented Nov. 21, 1944 Robert B. Dome, Bridgeport, Conn, assignor to General Electric Company, a corporation of .New York Application November 1, 1943, Serial No. 508,630

(c1. its-7.5)

My invention relates to translating circuits for television receivers and the like and particularly to circuits for detecting and amplifying television signals. I

It is an object of my invention to. provide a translating circuit for television signals including an improved arrangement for providing auto,- matic gain control with a minimum number of circuit components. It is another objeet of my invention to provide .a translating circuit for television signals ineluding an improved arrangement for securing automatic gain control and separation of synchronizing pulses. a I The features of my invention whichI believe to be novel are set forth with particularity in the appended claims. My invention itself, however,

both as to its organization and method of opera tion,'together with further'objects andadvantages thereof, maybest be understood by reference to the following description'taken in connection with the accompanying drawing, the single figure of which represents diagrammatically one form of signal translating circuit embodying my invention.

Briefly, the circuit illustrated in the accompanying drawing comprises a rectifier having a load resistance across which the television signal is reproduced in a positive sense and an electron discharge device for amplifying the signal. A grid resistor for the electron discharge device is connected in series with the load.resistor of the rectifier. The voltages across the two resistors are added to provide a voltage suitable for automatic gain control, the voltage across the load resistor being proportional to the instantaneous strength of the signal, that is, proportional. to the instantaneous energy transmitted,

and that across the grid resistor being a unidirectional voltage produced by grid rectification tween the plate of the device II indicated at I! and one side of the secondary transformer III.

" The plate isconnected to ground through acondenser l4 and a resistor I5. the functions of i which will be described below. The source of the signals impressed across the primary of the transformer I I! may be the last stage of anintermediate frequency amplifier and a condenser I8 is connected across the resistance I2 to bypass currents at intermediate frequencies. The signal is produced across the resistor I2 and is impressed in a positive sense upon a control electrode 01'' grid I8 of an electron discharge device IS; the signal is impressed on the grid I8 across a grid resistance 20 connected between the grid I8 and the grounded cathode 2I of the tube I9. The tube I9 includes an anode 22 connected to a suitable source of voltage through a resistor 23, an inductance 24 and a resistor 25, the resis-. tor 25 and the source being bypassed for alternating current by a suitable condenser 26. The output of the amplifier may be connected to a suitable utilization device coupled to the anode 22 in the usual manner.

In order to provide automatic gain control and maintain the volume of the output signal sub- I stantially constant over a wide range of intensi- Referring now to the drawingya. radio fre- 1 quency signal modulated by a television signal comprising picture signals and periodically reties of the input signal, an arrangement is pro-' vided for adding the voltages appearing across j the resistances l2 and 20. This arrangement comprises a. resistance 28 and a condenser '29 connected as a filter across the condenser I4 and resistance I6 in series, a voltage suitable for automatic volume control being available across the condenser 29. The connections for utilizing the automatic gain control voltage form no part of the present invention and have not been illustrated; the voltage may, of course, be employed in the usual manner by feeding it back to the intermediate amplifier stagesin a sense to reduce the amplifications thereof in response to increase in the received signal intensity.

The voltage or signal appearing across the resistance I2 is proportional to the instantaneous intensity of the input signal and is relatively high for a black picture and low for a white picture. The condenser I4 and the resistance 20 operate as a grid condenser and leak and produce a direct current bias voltage for the grid I8 due to grid rectification, the current being drawn The . ing impulses at the tops of the blanking signals.

The time constant'of the resistance 20 and condenser II is selected so that it is long with respect to'the interval between the synchronizing impulses so that a steady voltage is built up across the resistance 20 which is proportional to the difference between the instantaneous intensity of the signal and the intensity of the synchronizing impulses. For a black picture this voltage is relatively low and for a white picture relatively high. It will thus be apparent. that by adding the instantaneous voltages across the resistance 20 and across the resistance i2 a voltage is provided, the sum of which is proportional to the intensity of the peak of the input signal and is suitable for automatic gain control. The' time constant of the resistance 28 and condenser 29 are selected to smooth out residual variations due, for example, to varying picture content in the sum of the voltages across the resistances l2 and 20. It will thus be apparent that I have provided an arrangement for securing an automatic volume control voltage without the necessity of including an additional rectifier tube and circuit.

when the grid I8 draws current, the device l9 constitutes substantially a short circuit across the resistance and, consequently, a substantial part of the voltage across the load resistor Ills impressed across the resistance l5. Since the grid.

l8 draws current only during the peaks of the signal, that is, during the synchonizing impulses, these pulses may be taken oil by an output connected across the resistance IS. The separation of synchronizing impulses and picture signals is thus obtained without the necessity of providing an additional clipper circuit. A condenser 30 may be connected across the resistor ii to bybeing evident that picture signal components must pass through the resistance I5 in operation of the circuit; howevenonly the highest frequency components of thepicture signal may be objectional in the synchronizing pulse circuit,

is produced a voltage proportional to the instantaneous intensity of the rectified signal, means for amplifying the rectified signal and including a circuit element across which is produced a voltage proportional to the excess of the intensity of the synchronizing impulses over the instantaneous intensity .of the signal, and means for adding said voltages to produce a voltage proportional to the peak strength of the received signal and suitable for controlling the gain of the receiver.

2. A signal translating circuit for a television receiver or the like comprising means for the reception of a composite signal comprising picture signals and periodically recurring synchronizing impulses, the synchronizing impulses having greater amplitude than the picture signals of the same polarity, means including a first electron discharge device connected to rectify the received signal, means including a second electron discharge device having an anode and a cathode and a control electrode for amplifying the rectified signal, a. resistance and capacitance load circuit associated with said first device to provide a voltage. proportional to the instantaneous intensity of the received signal, a resistance and capacitance circuit associated with said second device for providing a voltage proportional to the excess of the intensity of the synchronizing impulses above the instantaneous intensity of the signal, and means for adding said voltages for providing a voltage proportional to the peak strength of the received signal andwhich is suitable for controlling the gain of the receiver.

- pass high frequency components to ground; it

and the condenser 30 may not be desirable in every application. The resistance 15 and condenser 30 may, of course, be. omitted if it is desired to provide other circuits for securing the synchronizing impulses because the resistance 15 and condenser 30 are rot necessary to the operation of the automatic volume control feature.

It will be apparent from the foregoing that 1 have provided a simple and eil'ective arrangement for securing automatic volume control and synchronizing pulse separation in a video frequency detector and amplifier system without the necessity of providing separate electron discharge devices and circuits for performing these additional functions.

Although I have illustrated one embodiment of my invention employing a diode detector and a three-electrode amplifier tube, other modifications will readily occur to those skilled in the art. I do not, therefore, desire my invention to be limited to the particular circuit arrangement disclosed herein, and I intend by the appended claims to cover all modifications which fall within the spirit and scope of my'inventiom What I claim as new and desire to secure by ture signals and periodically recurring synchronizing impulses, the synchronizing impulses having greater amplitude than the picture signals of the same polarity, means for rectifying said signal and including a circuit element across which 3. A signal translating circuit for a television receiver or the like, comprising means for the reception of a composite signal comprising picture signals and periodically recurring synchronizing impulses, the synchronizing impulses having greater amplitude than the picture signals of the same polarity, means including a first electron discharge device and a load resistance therefor for rectifying the received signal, a voltage proportional to the instantaneous intensity of the received signal appearing across said resistance, means including a second electron discharge devic'e having an anode and a cathode and acontrol electrode coupled to said resistance for amplifying the rectified signal, a resistance and capacitance circuit associated with said second device and dependent upon the drawing of current by said control electrode for providing a voltage proportional to the excess of the intensity of the synchronizing impulses above the instantaneous intensity of the rectified signal, and means for adding said voltages to provide a voltage proportional to the peak strength of the received signal and which issuitable for controlling the gain of the receiver.

4. A signal translating circuit for a television receiver or thelike comprising means for the reception of a composite signal comprising picture signals and periodically recurring synchronizingimpulses having greater amplitude than the electrode signals of the same polarity, a diode and a load resistance therefor for rectifying the signal, means including an electron discharge device having an anode and a cathode and a control grid coupled to said load resistance for amplifying the rectified signal, said last mentioned means including a grid resistance connected be- /tween said grid and said cathode, a third resistance connected between said cathode and said load resistance whereby said three resistances are connected in a closed series circuit, and an output connected across said third resistance for 'providing synchronizing impulses separated from having an anode and a cathode and a control grid connected to said resistance for amplifying the rectified signal, means including a second resistance connected between said grid and said cathode for providing a voltage proportional to the excess of the intensity of the synchronizing impulses above the instantaneous intensity of the rectified signal, a third resistance connected between said cathode and said loadresistance whereby said three resistances are connected in a closed series circuit, an output connected across 'said third resistance for providing synchronizing impulses separated from the picture signal, said synchronizing impulses being produced when current is drawn by said grid, and means for adding the voltages across said load resistor and said second resistor for providing a voltage proportional to the peak strength of the received signal and'which is suitable for controlling the gain of the receiver.

- ROBERT B. DOME. 

